PURPOSE: To ensure safety in tunnel construction by dividing a ring guarder at a tip of a propulsion jack in the circumferential direction, providing a jack for back ring pressure support in a clearance, and freely retractably arranging a tip spreader.

CONSTITUTION: A tip ring guarder 5 of a jack 4 provided in a shield machine is divided in the circumferential direction, and a clearance is provided between adjacent divided ring guarders. A tip spreader 8 of an expansion and shrinkage jack for back ring pressure support 6 which is independent from the jack 4 and a concrete press jack 3 is freely retractably arranged in this clearance. The divided ring guarder 5 is retracted to a facing side together with the other jacks after placing and curing lining concrete while back ring pressure applied on the shield machine is supported by the jack 6. Consequently, tunnel construction can be done safely by the shield machine.